Trundling along the ice, a penguin seems to epitomize an exhausting clumsiness. But according to two experts in biomechanics, it's not the distinctive side-to-side waddling that makes penguins such inefficient walkers, but their short legs.

A common assumption in biomechanics has been that penguins' swaying shuffle squanders energy, and walking therefore requires greater effort. Measurements seemed to bear out this assumption; compared with other animals of the same size, the birds used twice as much energy to cover the same distance.

To test the notion that waddling is wasteful, University of California, Berkeley, grad student Tim Griffin and his colleague Rodger Kram of the University of Colorado in Boulder enlisted five emperor penguins from Sea World in San Diego. With a little coaxing the birds would toddle along a walkway fitted with a force platform, a device like a glorified bathroom scale that can measure forces along three axes: up and down, forward and back, and side to side. Based on these force measurements, Griffin and Kram calculated that without waddling, penguins would have to do more work to keep on trucking. That is, waddling actually helps them walk, the researchers explain in the December 21/28 issue of Nature.

So if swaying isn't consuming the energy, what is? Griffin and Kram hypothesize that the penguins are handicapped by very short legs, which they must move very quickly with less efficient fast-twitch muscle fibers.

"The idea that waddling can be useful is amazing," says functional morphologist Stephen Gatesy of Brown University in Providence, Rhode Island. He adds that it might be possible to confirm Griffin and Kram's hypothesis by following the movements of the penguin's legs, though "it's hard to measure what the legs are doing under all that fat."

In other penguin news, a French team writing in the same issue of Nature has discovered that male king penguins can store food in their stomach for up to 3 weeks. This allows the fathers to feed their chicks long after returning from fishing trips in the ocean. Go penguins.